The present invention relates to the transmission and control of electromagnetic energy and, more particularly, to an optoelectronic device utilizing nanometer-scale particles.
In recent years, designers of sophisticated, high speed electronic circuits have become interested in the use of optical signaling, both for its speed and because it virtually eliminates energy loss due to heat. Prior optical systems depend on the principle of total internal reflection, however, making them unsuitable for use when the lateral dimension of a waveguide falls below xcex/2n, which for visible light in glass is approximately 250 nanometers. In many miniature electronic circuits, such as modern high speed microprocessors, the size of electrical conductive paths is already less than half this limit. Also, there are limitations on how sharply conventional optical waveguides can be bent before their transmission properties are lost. Continuous conductive paths are also limited by their inherent electrical resistance when they are made very small. Thus, it is desirable to develop a form of interconnect which can be made much smaller than the diffraction limit and can be formed into arbitrary patterns without sacrificing efficiency.
In a somewhat different field, nanometer-scale particles (xe2x80x9cnanoparticlesxe2x80x9d) have been fabricated and given a degree of order using a technique known as xe2x80x9cself-assembly.xe2x80x9d By inducing repulsive forces on close approach and longer range attractive forces on particles in colloidal suspensions, such particles are readily brought into closely packed arrays. An example of this is the colloidal synthesis and arrangement of silica-coated gold particles described by L. M. Liz-Marzan, M. Giersig, and P. Mulvaney in Langmuir 12:4329 (1996). In this way, control has been obtained over the orientation of the particles and the spacings between them. Metal colloids useful for this purpose can be obtained from a number of commercial sources.
Linear chains of nanoparticles have been observed in suspensions of polarizable particles subjected to electric fields or magnetizable particles subjected to magnetic fields. Fermigier and Gast, J.Magn.Mater.122:46 (1992), confined paramagnetic particles in a narrow channel and examined the structures of the particulate agglomerates that resulted when magnetic fields of different strengths were applied. Induced dipoles caused the particles to be attracted to one another in the direction of the applied field and to repel one another in the orthogonal direction. The addition of surfactant molecules to prevent aggregation facilitated reversible dipole-induced ordering. Individual chains of such particles were not obtained, however, and the arrays were not suggested for use in the transfer of electromagnetic energy.
Thus, a need exists for structures capable of efficiently transferring and controlling electromagnetic energy below the diffraction limit. The present invention addresses these needs.
The present invention is directed to an optoelectronic device and related method in which nanoparticles are arranged in chains or xe2x80x9cwiresxe2x80x9d for the transfer and control of electromagnetic energy in a variety of circuit configurations. The particles are spaced apart along a preselected path such that electrical polarization of one of the particles acts to induce a corresponding polarization in an adjacent one of the particles. These devices rely on near-field interaction between the nonoparticles to set up coupled polarization or plasmon modes. Although the coupling is believed to be primarily of dipoles created in the particles, in some cases the coupling can be part dipole and part higher order multiple.
The particles of a chain may include metals, semiconductors or other materials capable of being polarized, and polarization of a first particle may be created by light. The particles themselves may be individual atoms or molecules, or aggregations of atoms and/or molecules, and may be separated by dielectric particles or coated with dielectric materials to create the required spacing. In one embodiment, the particles are metallic and interact with each other through coupling of plasmon modes.
In a another embodiment, the optoelectronic device of the invention extends from an input device, which may be a light source, to an output at a terminal portion of a chain. The output device may be a detector of light or any other electromagnetic output of the chain, or it may be a waveguide or other suitable device. The spacing between particles may be uniform or nonuniform, and the chain may form arbitrary angles without affecting the efficiency of energy transmission.
The device may function as a switch, a filter or other suitable device, depending on configuration and use. Specifically, a first group of nanoparticles may extend along a primary path from a first end to a second end of the device, and a second group of nanoparticles may extend along a secondary path intersecting the primary path between the ends of the primary path, to modulate propagation of a polarization signal along the primary path. Modulation occurs by constructive or destructive interference between signals along the two paths, and it is possible to null the signal on the primary path by appropriate choice of the magnitude and polarization of the modulating signal.